How furosemide works
Like all loop diuretics, furosemide is a so-called “high-ceiling diuretic”. With such diuretics, water excretion can be increased in proportion to the dose over a wide dose range. This is not possible with other diuretics (e.g. thiazides). Here, a maximum effect sets in after a certain dose, which cannot be intensified by further dose increases.
The blood is filtered in the kidney. Waste products, pollutants and also some drugs are filtered out and ultimately excreted in the urine. The smallest functional unit in the kidney is the nephron, which consists of the renal corpuscle and a renal tubule.
Nephrons filter small molecules from the blood (blood proteins and blood cells remain in the blood). The resulting primary urine is still unconcentrated and is concentrated in the renal tubules by reabsorption of the water it contains. In the process, other substances that are important for the body can also be filtered out and reabsorbed into the blood (for example, glucose, sodium, potassium and chloride ions).
Together with these charged particles, large amounts of water are also excreted, which is the actual intended furosemide effect. When furosemide is administered in high doses, urine volumes of up to 50 liters per day are possible. The increased excretion of water causes blood pressure to drop and reduces water retention in the body.
Absorption, breakdown and excretion
After ingestion, about two-thirds of furosemide is absorbed from the intestine into the blood. The effect occurs after about half an hour.
Only a small portion of the active ingredient is metabolized in the liver (about ten percent); the remainder is excreted unchanged – about one-third in the stool, the remaining amount in the urine. After about one hour, half of the active substance has been excreted.
When is furosemide used?
Furosemide is used for:
- water retention in the body (edema) due to diseases of the heart, kidneys, or liver
- impending kidney failure (renal insufficiency)
Depending on the underlying disease, the active ingredient is taken only for a short time or as long-term therapy.
How furosemide is used
In most cases, dosages of 40 to 120 milligrams of furosemide per day are sufficient. However, in individual cases and depending on the underlying disease, the attending physician may prescribe dosages of up to 500 milligrams a day.
In hypertension therapy, furosemide can be combined with other antihypertensive drugs to reduce the side effect rate and increase the effectiveness of the treatment.
What are the side effects of furosemide?
In more than one in ten patients, side effects include electrolyte disturbances (especially altered sodium and potassium levels), fluid deficiency, low blood volume and blood pressure, increased blood lipid levels, and increased blood creatinine levels.
In addition, one in ten to one in a hundred patients experience elevated blood cholesterol and uric acid levels, gout attacks, and symptoms resulting from electrolyte disturbances (calf cramps, loss of appetite, feeling weak, drowsiness, confusion, cardiac arrhythmias, etc.).
The risk of electrolyte disturbances and fluid deficiency is greater in elderly patients than in young ones.
What should be considered when taking furosemide?
Contraindications
Furosemide must not be used in:
- Renal failure that does not respond to furosemide therapy.
- Hepatic coma and precursor thereof (coma hepaticum, praecoma hepaticum) associated with hepatic encephalopathy, i.e., brain dysfunction due to inadequate detoxification by the liver
- Hypokalemia (low potassium level)
- Hyponatremia (low sodium level)
- Hypovolemia (decreased amount of circulating blood volume) or dehydration (dehydration)
Drug Interactions
If certain other agents such as glucocorticoids (“cortisone”) or laxatives are taken during therapy with furosemide, this may result in low potassium blood levels. The same is true if the patient consumes large amounts of licorice.
Non-steroidal anti-inflammatory drugs (such as ASA), which are often used as painkillers, for example, can weaken the effect of furosemide. The same effect may occur with combined use of phenytoin (for epilepsy) or agents that are also excreted through the renal tubules, such as probenecid (for gout) and methotrexate (for cancer and autoimmune diseases).
The simultaneous use of furosemide and agents that damage the kidneys or hearing (nephrotoxic or ototoxic effect) should be avoided. Examples of such agents include antibiotics such as gentamycin, tobramycin, kanamycin, and anticancer drugs such as cisplatin.
Concomitant use of the mood stabilizer lithium should only be closely monitored because lithium is transported in the body like sodium. Furosemide may therefore significantly alter its distribution in the body.
Age restriction
Furosemide is also suitable for the treatment of children, but at an appropriately reduced dosage. Since children under the age of six often have problems swallowing tablets, the oral solution should be used in this case.
Pregnancy and lactation
Furosemide crosses the placental barrier and can therefore pass into the unborn child. During pregnancy, the diuretic should therefore only be used under strict medical supervision and only for a short time.
The active substance passes into breast milk, which is why nursing mothers should stop breastfeeding.
How to obtain medication with furosemide
Since when is furosemide known?
Starting in 1919, toxic mercury compounds were used as diuretics. In 1959, the mercury-free active ingredient furosemide was finally developed as an alternative. A patent application was filed for it in 1962 and it was soon used in practice.
